Microporous membranes are well known in the art. Microporous membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous membrane become trapped on or in the membrane structure effecting filtration. A slight pressure, generally in the range of about 5 to 50 psig (pounds per square inch gauge) is used to force fluid through the microporous membrane. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores. The liquid and particles smaller than the pores of the membrane pass through. Thus, a microporous membrane prevents particles of a certain size from passing through it, while at the same time permitting liquid and particles smaller than that size to pass through. Microporous membranes have the ability to retain particles in the size range of from about 0.01 to about 10.0 microns.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about 8 microns in diameter, platelets are about 2 microns in diameter and bacteria and yeasts are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry. Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out all over the membrane (foam-all-over-point or "FAOP"). The procedures for conducting initial bubble point and FAOP tests are well known in the art. The procedures for these tests are explained in detail for example in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference. The bubble point values for microporous membranes are generally in the range of about 5 to about 100 psig, depending on the pore size.
U.S. Pat. No. 3,876,738 describes a process for preparing microporous membranes by quenching a solution of a film-forming polymer in a non-solvent system for the polymer. U.S. Pat. No. 4,340,479 generally describes the preparation of skinless microporous polyamide membranes by casting a polyamide resin solution onto a substrate and quenching the resulting thin film of polyamide.
Since the mechanical strength of some microporous membranes is poor, it is known to reinforce such membranes with a porous support material to improve mechanical properties and facilitate handling and processing. Accordingly, the aforementioned U.S. Pat. No. 4,340,479 describes a procedure wherein polymer solution is directly cast onto a porous support material so that the polymer solution penetrates the support material during casting and becomes firmly adhered thereto during formation of the composite microporous membrane. The support material preferably possesses an open structure so that pressure drop across the composite membrane is minimized. U.S. Pat. No. 4,340,479 further discloses combining two microporous membranes, one of which may be reinforced, to form a dual layered structure which is dried under conditions of restraint to produce a single sheet having particle removal characteristics superior to those of individual layers.
U.S. Pat. No. 4,707,265 discloses a reinforced laminated filtration membrane comprising a porous reinforcing web impregnated with a polymeric microporous inner membrane and at least one polymeric microporous outer qualifying membrane laminated to each side of the impregnated web. The pore size of the inner membrane is greater than the pore size of the outer membranes. In this manner, the imperfections, e.g., fiber bundles, broken fibers, void areas, and the like, which are invariably present in the reinforcing web are confined to a coarse, more open inner membrane and the tighter outer qualifying layers are strengthened and supported by the web. The qualifying layers are not affected by imperfections present within the reinforcing web. Further, the use of a coarse, large pore size inner membrane layer insures that there is no substantial pressure drop of fluid across the reinforcing web.
The membranes disclosed in U.S. Pat. No. 4,707,265 are complicated and costly to produce since three separate operations are required to produce the composite membrane: first, the impregnated reinforced membrane support layer is produced, second, the non-reinforced qualifying layers are produced and, third, the impregnated reinforced membrane support layer and the non-reinforced qualifying layers are laminated to form the multilayer composite microporous membrane.
Due to processing and handling restraints, there is a limit to how thin the impregnated reinforced membrane support layer and the non-reinforced qualifying layers can be. As a result, the multilayer composite microporous membrane of U.S. Pat. No. 4,707,265 is at least about 10 mils thick. Furthermore, the overall pore size of the composite membrane described in U.S. Pat. No. 4,707,265 is generally limited to the range of approximately 0.45 microns or higher due to the difficulties of producing and handling non-reinforced qualifying layers having pore sizes of as low as 0.45 micron. Thus, the utility of the composite membrane is limited to nonsterilizing applications and other applications where membranes having 0.65, 0.8, 1.2, 3.0 and greater micron ratings are acceptable.
As the thickness of a membrane increases, pressure drop increases, flow rate worsens and the performance characteristics of the membrane are adversely affected. For example, with increasing thickness the total number of pleats in a pleated cartridge element decreases, thereby reducing the effective surface area available for filtration. Furthermore, a mechanical strain exists at the crest of each pleat and increases with increasing thickness. As a result, thick membranes are more likely to crack during the pleating, edge-seaming, etc. operations that are attendant to the production of pleated filter cartridge elements or during oxidative hydrolytic exposure or multiple steam cycling. Therefore, mechanical strains, which can never be fully relieved after cartridge fabrication, decrease the useful life of the product and lead to early failure in integrity.
U.S. Pat. No. 4,770,777 overcomes some of the shortcomings of the process disclosed in U.S. Pat. No. 4,707,265 by completely impregnating the reinforcing web with a large pore size (coarser) membrane casting solution, applying a small pore size membrane casting solution to one side of the coated web and then simultaneously quenching the large and small pore size casting solutions to provide a continuous, geometrically asymmetric membrane possessing a pore size gradient. Thus, the lamination step of U.S. Pat. No. 4,707,265 is eliminated, along with the necessity of handling the fragile non-reinforced qualifying layers. However, the membrane produced in U.S. Pat. No. 4,770,777 is skinned. Accordingly, the membrane suffers from drawbacks associated with skinned microporous membranes, in particular, high pressure drop, poor structural integrity, susceptibility to skin breach, propensity to becoming fouled by debris, etc.
U.S. Pat. No. 5,433,859 attempts to address some of the deficiencies, in particular, high pressure drop, of the skinned membrane disclosed in U.S. Pat. No. 4,770,777 by proposing an incomplete impregnation of the reinforcing web with coarse membrane casting solution so that a portion of the reinforcing web having a thickness of about 50 microns is not embedded within the continuous microporous membrane. The low flow resistance of that portion of the reinforcing web which is not embedded within the microporous membrane ensures that filtered fluid passing through the supported microporous membrane will not have a significant adverse impact on the pressure drop across the filtration element.
While the membrane disclosed in U.S. Pat. No. 5,433,859 exhibits lower pressure drop across the membrane compared to the skinned membrane disclosed in U.S. Pat. No. 4,770,777, the membrane does have significant structural drawbacks. First, the membrane suffers from tremendous geometric asymmetry around the central axis of the reinforcing web, i.e., the thickness of the membrane varies on each side of the reinforcing web. As a result, when the membrane is pleated, the mechanical strain on the thick side of the membrane is greater than on the thin side of the membrane. This differential in mechanical strain increases the possibility of stress crack formation and failure of the integrity of the membrane Second, the membrane poses a high risk of delamination along the membrane-reinforcing web interface, especially during backwashing operations. Third, the membrane can only be used with the open pore side of the membrane facing upstream, i.e., the membrane exhibits "sidedness".